The invention relates to a process for suppressing total echoes comprised of a plurality of partial echoes in telecommunications (TK) equipment such as terminals, transmission systems, or switching equipment by means of adaptive FIR (=Finite Impulse Response) filters that simulate the total echo and subtract it from the echo-affected useful signal transmitted to the respective TK equipment, whereby n partial FIR filters are provided, which respectively simulate a partial echo.
When voice signals are transmitted on telecommunication lines, acoustic echoes can form, on the one hand, at the xe2x80x9cnear endxe2x80x9d of the sending user due to the direct sound transmission from the loudspeaker to the microphone of the terminal. Particularly if several terminals are arranged in close spatial proximity to each other, for example, in an office or conference room with many telephone connections, the near echo problem is significantly increased since there is a coupling from each loudspeaker signal to each microphone. A multi-channel echo suppression process proposed in EP 0 627 825 A2 and the associated electronic circuitry is intended to provide a remedy. However, in addition to the near acoustic echo, an electric line echo, possibly composed of a variable number of partial echoes, also forms due to the reflections of the sent voice signals at variably remote points in the transmission channel, e.g. with 2-4 wire hybrids (terminating sets) in the terminal or in the witching equipment and gateways. A distinction is typically drawn between so-called xe2x80x9cnear echoesxe2x80x9d that are reflected back to the speaker within a time interval of up to 128 ms and far echoes, which return to the speaker within a time frame of 640 ms, the large skew of which is particularly disturbing since the speaker has already made noticeable progress in his speech by the time his own reflected voice signal comes back to him. Such delayed far echoes occur, for example, in intercontinental calls transmitted via deep-sea cables or satellites. The network operators maintaining the corresponding long-distance networks therefore aim to suppress, as effectively as possible, particularly the far echoes with a large skew as well as the near echoes, which may be reflected back with greater intensity.
EP 0 792 029 A2, for example, proposes an echo suppression device with an adaptive filter comprised of a coarse and fine detector for near-end voice signals arranged on both sides of the echo suppression device to permit monitoring of the incoming signal before and after echo suppression. During speech pauses from the near end, this is to permit an adaptation to the reflected echo value.
The initially cited DE 44 30 189 A1 proposes a cost-effective process which can be used under different acoustic conditions and which uses a FIR filter whose filter coefficients are determined based on the NMLS algorithm. For echo suppression in a TK network with many parallel channels, this method uses such an echo suppressor with an adaptive FIR filter implemented in software in a digital signal processor for each channel. For this purpose, the adaptive FIR filter is to simulate the total echo to be expected, possibly comprising several individual echoes, and to subtract the echo-affected useful signal transmitted to the TK terminal.
In contrast, the object of the present invention is to further develop a process of the initially described type, which permits effective echo suppression without requiring excessive memory space and computation effort, even if there is a very large number of m parallel, mutually independent TK channels, such as must be processed, for example, in switching equipment or network hybrids.
This object is attained by the invention in that m parallel, mutually independent TK channels are provided with an echo suppression function and that a variable sub-quantity of the partial FIR filters is assigned, respectively, to a TK channel to be operated as required, depending on the number of partial echoes and the size of the occurring delay times between signal and echo.
This flexible assignment of the partial FIR filters per processed TK channel as a function of the number of individual echoes and the size of the occurring delay times between signal and echo makes it possible, despite the very high number of m channels, to design the system for implementing the process in a compact manner and to utilize it to a high degree.
Particularly preferred is an embodiment of the process according to the invention, wherein the m independent TK channels are operated in time division multiplexing with echo suppression functions. This permits an even-compacter hardware or software design to implement the process, since many functional elements, e.g. partial FIR filters, can be used simultaneously for several TK channels. In a further advantageous embodiment, the adaptive FIR filters are implemented by a computer program that calculates the partial echoes to be simulated in real time. On the one hand, this minimizes the hardware for the system for implementing the process according to the invention and, on the other hand, permits system changes in a simple manner through modification or replacement of the computer program. In a further development of this embodiment, the computer program comprises same-type subroutines that run independently and synchronously to each other, whereby each of these subroutines acts as a partial FIR filter and can simulate a partial echo. Such a modular program is particularly simple to create and, in practice, can be expanded to any number of parallel running subroutines of the same type and thus to a corresponding number of partial FIR filters.
An alternative embodiment to the above described software solution provides that n same-type partial FIR filters capable of simulating partial echoes, respectively, are implemented in a single ASIC (=Application Specific Integrated Circuit); that the partial echoes to be simulated are calculated in real time in the ASIC; and that a digital signal processor is provided to control the ASIC and adjust the filter settings, particularly after calculation of the coefficients required for echo simulation and time delays in the ASIC.
The high integration density of an ASIC keeps a device for implementing the process according to the invention particularly compact. Since the digital signal processor is significantly relieved by integrating the FIR filters into the ASIC, many more parallel TK channels can be operated with effective echo suppression at a processor effort that remains constant. This permits processing of even larger echo delay times (e.g. up to 640 ms), many individual echoes per total echo, and many parallel channels ( greater than 2000) at minimum cost. Advantageously, a number of partial FIR filters are operated in cascaded manner to simulate a total echo. Currently, TK network operators require three to five partial echoes to be suppressed per channel. This number may further increase in the future.
An advantageous further development is distinguished by the fact that, for each partial FIR filter, a macrocell is provided in the ASIC comprising a shift register of suitable length for intermediate storage of the sampled voice signals, a demultiplexer unit for adjusting the effective shift register length, and a summing unit for adding the sampling values weighted with the corresponding coefficients. The macrocells can be identically configured to simplify production and thus reduce the cost of an ASIC with many same-type partial FIR filters.
In particular, cascading of a plurality of macrocells is easily possible, which permits any desired, flexible interconnection, e.g. for simulating a total echo, and thus a particularly high utilization of the macrocells.
A further variant of the process for echo suppression according to the invention is distinguished in that a Dirac impulse is sent to the corresponding TK channel, the echo answer thereto is detected, and suitable coefficients for simulating the total echo are calculated and stored for the corresponding FIR filter.
Instead of the Dirac impulse, other process variants send a defined analog noise signal sequence to the corresponding TK channel, detect the echo answer thereto, and calculate and store suitable coefficients for simulating the total echo. The send duration of the noise signal sequence is typically less than one second.
Particularly preferred is a further development of this process variant, in which the analog noise signal sequence comprises a Gaussian noise signal that is preferably limited to the bandwidth of the TK channel to be operated.
A further alternative process variant is distinguished in that a synthetic, preferably ternary pseudo noise sequence is sent to the corresponding TK channel, the echo answer thereto is detected, and suitable coefficients for simulating the total echo are calculated and stored. As a rule, the pseudo noise sequence is selected such that only the main value of the sequence has a correlation value q, while the subvalues have the correlation value 0.
It is particularly preferred to obtain the coefficients for echo simulation by correlating the sent signals with the received echo signals.
Also preferred is a further development of the process according to the invention, in which a NLMS (=Normalized Least Mean Square) algorithm is used for calculating the coefficients from the received echo signals acknowledging the sent signals. This minimizes the required computing time and computing capacity to achieve a satisfactory result. In this case, the length of the noise sequence is advantageously matched to the length of the FIR filter.
In a further preferred process variant, the transmission of non-voice signals on a TK channel is additionally detected and, particularly if modem or fax signals are identified, the echo suppression function on the corresponding TK channel is automatically switched off. This increases connection reliability for fax or modem connections.
In a further development of this process variant, a discrete Fourier transformation (=DFT), particularly a Goertzel algorithm, is used for detecting non-voice signals. As a result, this additional function requires very little memory space and computing capacity and detection is very fast and reliable.
In addition, in a further advantageous process variant, a compander function may also be provided for combined suppression and masking of line echoes by sending the receive level on the corresponding TK channel during speech pauses. This further enhances echo suppression and thus increases the comfort for the end subscriber.
The compander function can also be implemented as a computer program or in the form of hardware in an ASIC, preferably the same ASIC as the partial FIR filters. In this case, the hardware units are particularly compact and inexpensive to manufacture.